1. Field of the Invention
This invention relates to a semiconductor integrated circuit device, and particularly to a protection circuit for providing protection against noise such as static electricity or the like applied to an input terminal or an output terminal of an integrated circuit device.
2. Description of the Related Art
In a conventional protection circuit for providing protection against the input of noise such as static electricity or the like, the source of a protective N channel MOS transistor (hereinafter abbreviated merely as "NMOS transistor") is electrically connected to an input terminal, the drain thereof is electrically connected to a power source terminal and the gate thereof is electrically connected to a ground terminal. The conventional protection circuit protects an internal circuit against the noise, using a breakdown in the NMOS transistor.
When positive pulse-shaped voltage noise (e.g., 1000V) is applied to the input terminal through an external pad due to static electricity or the like, the protective NMOS transistor breaks down. Thus, current flows from the source thereof electrically connected to the input terminal to the drain thereof electrically connected to the power source terminal. Namely, since the noise is discharged from the input terminal to the power source terminal, the internal circuit is protected from the noise.
On the other hand, when negative voltage noise (e.g., -1000V) is applied to the input terminal due to the static electricity or the like, the protective NMOS transistor is turned ON. As a result, current is produced and flows from the drain thereof electrically connected to the power source terminal of the protective NMOS transistor to the source thereof electrically connected to the input terminal. Therefore, since the noise is substantially discharged from the input terminal to the power source terminal, the internal circuit is protected from the noise.
When a negative voltage exceeding the threshold value of the protective NMOS transistor is applied to the input terminal in the conventional protection circuit, a current flows from the power source terminal of the corresponding integrated circuit device to the input terminal thereof. At this time, the neighborhood of the drain of the protective NMOS transistor is brought to a high electric field. Hot electrons produced due to the high electric field will cause impact ionization, thereby allowing a substrate current to flow between the drain thereof and a substrate. As a result, the potential applied to the substrate of the integrated circuit device increases. This will cause interference with the stable operation of the integrated circuit device. Therefore, improvements in this point have been desired.